This invention relates to a process for forming a metallic film on a surface of an inorganic material such as ceramic useful in the production of printed wiring boards.
As a process for forming a metallic film such as circuit conductors on ceramic substrates, there have been known a so-called thick film method and a so-called thin film method. According to the thick film method, a conductor paste which is a mixture of glass and a metal is printed on a ceramic substrate and baked to form a conductor pattern. This method is advantageous in that strong bonding between the conductor and the ceramic can be obtained by the glass bonding with a relatively inexpensive manner, but disadvantageous in that the conductor resistance is high, the circuit density cannot be enhanced due to printing, and the conductor line shape is bad and unsuitable for high-frequency circuits. On the other hand, according to the thin film method, a metal is cladded on a ceramic substrate by vapor deposition or cathode sputtering. This method is advantageous in that a highly precise conductor pattern can be obtained due to the formation of the circuit pattern by photo etching and thus suitable for high-frequency utility, but disadvantageous in that the production cost is high due to the use of expensive apparatus.
In order to overcome the disadvantges mentioned above, there have been proposed processes for forming copper conductor patterns by conducting electroless plating of copper directly on a ceramic substrate, following by etching, and the like. These processes can be divided into (1) a process comprising treating a surface of a cermaic substrate (particularly an alumina substrate) with borofluoric acid, hydrofluoric acid, or the like to give a roughened surface, and conducting electroless copper plating and electroplating of copper, and (2) a process comprising treating a surface of a ceramic substrate with an alkali to give a roughened surface, and conducting electroless copper plating and electroplating of copper [Japanese Patent Unexamined Publication Nos. 47-11652, 54-82666, 58-104079]. According to process (1), there is a problem in that the adhesive strength between the plated copper and the ceramic is low. On the other hand, the roughening mechanism of ceramic substrate comprises a firsrt step of dissolving a vitreous substance richly distributed on a ceramic surface, a second step of roughening particles which constitute mainly a surface first layer (alumina particles in the case of alumina ceramics), and a third step of roughening and dissolving glass and particles of a second layer. But it is necessary to stop the roughening at the second step. The amount of glass richly distributed on the surface layer changes depending on production lots or manufacturers and the dissolving rate by the alkali is not constant, even if the composition may be the same. Therefore, the process (2) has a problem in that it is difficult to control the roughening amount or roughening time constant.